The market for polyethylene terephthalate (PET) refillable carbonated soft drink (CSD) bottles has enjoyed significant growth worldwide, since its introduction in 1987 by Continental PET Technologies. These bottles have been commercialized throughout much of Europe, Central and South America, and are now moving into the Far East market.
Refillable bottles reduce the existing landfill and recycle problems associated with disposable plastic beverage bottles. In addition, a refillable bottle provides a safer, lighter-weight plastic container in those markets, currently dominated by glass, where legislation prohibits use of non-returnable packages. The goal is to produce a refillable bottle having the necessary physical characteristics to withstand numerous refill cycles, while being economical to produce.
Generally, a refillable plastic bottle must maintain its functional and aesthetic characteristics over a minimum of 10 and preferably 20 cycles or loops to be economically feasible. A cycle (illustrated in FIG. 3) is generally comprised of (1) an empty hot caustic wash, (2) contaminant inspection (before and/or after wash) and product filling/capping, (3) warehouse storage, (4) distribution to wholesale and retail locations and (5) purchase, use and empty storage by the consumer, followed by eventual return to the bottler.
Refillable containers must fulfill several key performance criteria in order to achieve commercial viability, including:
1. high clarity (transparency) to permit visual on-line inspection; PA1 2. dimensional stability over the life of the container; and PA1 3. resistance to caustic wash induced stress cracking and leakage.
A commercially successful PET refillable CSD container is presently being distributed by The Coca-Cola Company in Europe (hereinafter "the prior art CSD container"). This container is formed of a single layer of a polyethylene terephthalate (PET) copolymer, with 3-5% comonomer such as 1,4-cyclohexanedimethanol (CHDM) or isophthalic acid (IPA). The preform, from which this bottle is stretch blow molded, has a sidewall thickness on the order of 5-7 mm, or about 2-2.5 times that of a preform for a disposable one-way bottle. This provides a greater average bottle sidewall thickness (i.e., 0.5-0.7 mm) required for abuse resistance and dimensional stability, based on a planar stretch ratio of about 10:1. The average crystallinity in the panel (cylindrical sidewall section beneath the label) is about 15-20%. The high copolymer content prevents visual crystallization, i.e., haze, from forming in the preform during injection molding. Preform haze is undesirable because it produces bottle haze which hinders the visual on-line inspection required of commercial refill containers. Various aspects of this prior art container are described in Continental PET Technology's U.S. Pat. Nos. 4,725,464, 4,755,404, 5,066,528 and 5,198,248.
The prior art CSD container has a demonstrated field viability in excess of 20 refill trips at caustic wash temperatures of up to 60.degree. C. Although successful, there exists a commercial need for an improved container that permits an increase in wash temperature of greater than 60.degree. C., along with a reduction in product flavor carryover. The latter occurs when flavor ingredients from a first product (e.g., root beer) migrate into the bottle sidewall and subsequently permeate into a second product (e.g., club soda) on a later fill cycle, thus influencing the taste of the second product. An increase in wash temperature may also be desirable in order to increase the effectiveness and/or reduce the time of the caustic wash, and may be required with certain food products such as juice or milk.
The above-identified "Related Applications," namely U.S. Ser. Nos. 07/909,961 and 08/082,171, describe an improved refillable container having a higher caustic wash temperature (above 60.degree. C.) and reduced product flavor carryover for a lifetime of 20 refill trips. The improved container is blow molded from a multilayer preform, having an interior layer of a first relatively high copolymer polyester and an exterior layer of a second polyester with relatively less copolymer and a crystallization rate at least 20% higher than the first polyester. The high copolymer prevents crystallization (haze) in the interior core layer during injection blow molding (which core layer cools more slowly), in order to preserve overall container clarity and flexibility. The exterior (inner and outer) layers are made from a homopolymer or low copolymer polyester which has a higher rate of strain-induced crystallization and thus provides enhanced resistance to caustic cracking and reduced flavor carryover at the surface of the blown container. The subject matter of these two applications is hereby incorporated by reference in their entirety.
Polyethylene naphthalate (PEN) is another potential polyester for use in blow-molded containers. PEN has a desirable oxygen barrier capability--about five times that of PET, and a higher heat stability temperature--about 250.degree. F. (120.degree. C.) compared to about 175.degree. F. (80.degree. C.) for PET. These properties would be useful in containers for oxygen-sensitive products (e.g., food or cosmetics) and/or containers subjected to high temperatures (e.g., refill or hot-fill containers). However, PEN is substantially more expensive than PET and has different processing requirements such that PEN has not been successfully used in a commercial blow-molded beverage container.